1. Field
The following description relates to training sequences and a channel estimation method for spread spectrum based systems.
2. Description of Related Art
An advancing form of wireless communication is spread spectrum communication. In spread spectrum communication, a frequency of a transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have had if its frequency was not varied. Spread spectrum techniques are used for a variety of reasons, such as establishing secure communications, multiple accesses, increasing a resistance to a natural interference, noise and jamming, preventing a detection, and limiting a power flux density.
Spread spectrum telecommunications is a signal structuring technique that employs a direct sequence, frequency hopping, or a hybrid of these, which can be used for multiple accesses and/or multiple functions. This technique decreases a potential interference to other receivers while achieving privacy. Spread spectrum makes use of a sequential noise-like signal structure to spread a narrowband information signal over a relatively wideband (radio) band of frequencies. A receiver correlates received signals to retrieve the original information signal. There are two intentions: either to resist enemy efforts to jam a communications (anti-jam, or AJ), or to hide the fact that the communication was even taking place, sometimes called low probability of intercept (LPI). Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum techniques. Each of these techniques employs pseudorandom number sequences created using pseudorandom number generators to determine and control a spreading pattern of a signal across an allotted bandwidth. Ultra-wideband (UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses.
Although, spread spectrum communications is efficient in many cases, there are several disadvantages, such as power levels of all user devices received at a base station having to be equal if bit rates are equal, and therefore, fast power control being needed. In other cases, user devices in a soft handover mode need resources of more than one cell, hence a system capacity may be reduced.
Channel estimation is a component of coherent communication systems. Channel estimation is an estimation of a frequency and/or time domain response of a path between a transmitter and a receiver to an impulse as an input. This can be used to optimize performance and maximize a transmission rate. Pilots or training symbols are transmitted, which are then used by the receiver for channel estimation. The use of such pilots adversely affects a bandwidth efficiency. The loss in the bandwidth efficiency is severe with a deployment of communication techniques like MIMO (Multiple-Input and Multiple-Output), where there is a need to estimate each of links. If a number of antennas increases, such as in large MIMO systems as in a next generation mm-Wave GHz communication like a 60 GHz Industrial, Scientific and Medical (ISM) band as in Institute of Electrical and Electronics Engineers (IEEE) 802.11ad and IEEE 802.15.3c and e-band, bandwidth resources allocated for pilots increases rapidly.
Further, for co-operative communications the problem is further accentuated as an estimate of links from a node and relays are needed. A channel root mean square (rms) delay spread also affects a bandwidth efficiency as a pilot design will need a sufficiency of excitation, which can lead to a loss in a bandwidth efficiency.
Multicarrier spread spectrum techniques, like MC-CDMA (multi carrier code division multiple access) and MC-DS-CDMA (multi carrier direct sequence code division multiple access), are attractive as they have an additional degree of freedom due to a use of spreading codes. Each of the spreading codes is a bit of information, such as a “1”, a “0”, a “−1”, or a “+1” (bipolar). It is anticipated that a next generation interface will evolve from orthogonal frequency-division multiplexing (OFDA)/OFDM Access (OFDMA) to MC-CDMA/MC-DS CDMA due to an additional flexibility offered by the spreading codes. MIMO and co-operative communications may be built on top of such multi carrier spread spectrum systems. Sub sampled OFDM/single-carrier frequency-domain-equalization (SC-FDE) based sub-band wideband systems for 60 GHz and UWB communication are special cases of multi carrier spread spectrum systems.
An alternative to multiplexed pilots is a superimposed pilots based method for channel estimation. In this method, pilots are added to data at a transmitter, and these are used at a receiver to estimate channel coefficients in a presence of interference and noise. A challenge in channel estimation methods is reducing an impact of noise and interference, ensuring a good estimation accuracy and also a good bandwidth efficiency.
Due to abovementioned reasons, existing superimposed training based channel estimation methods where interference is cancelled at a receiver end are time consuming iterative processes. Further, feedback based methods of interference cancellation are not able to cancel noise and data interference efficiently. Hence, there is a need for bandwidth efficient channel estimation while maintaining an estimation accuracy in spread spectrum systems. Further, in energy efficient communications, pilot overhead may need to be reduced.